OBJECTIVE While adenomyosis is one of the most common benign histologic findings in hysterectomy specimens of endometrial cancer, demographics of endometrial cancer arising in adenomyosis (EC-AIA) has not been well elucidated. The aim of this study is to evaluate histopathological findings and disease-free survival (DFS) of EC-AIA in comparison to endometrial cancer coexisting with adenomyosis (EC-A). METHODS EC-AIA cases were identified via a systematic literature search (n = 46). EC-A cases were identified from a historical cohort that underwent hysterectomy-based surgical staging in two institutions (n = 350). Statistical comparisons of the two groups were based on univariate and multivariate analyses. RESULTS The EC-AIA group was significantly older than the EC-A group (58.9 versus 53.8, p = 0.002). As to tumor characteristics, 63.6% of EC-AIA cases reported tumor within the myometrium without endometrial extension. The EC-AIA group was significantly associated with more non-endometrioid histology (23.9 versus 14.8%; p = 0.002) and deep myometrial tumor invasion (51.6 versus 19.4%; p < 0.001) than EC-A. Tumor grade, stage, and nodal metastasis risk were similar (all, p > 0.05). In a univariate analysis, the EC-AIA group had a significantly decreased DFS compared to EC-A (5-year rates, 72.2 versus 85.5%, p = 0.001). After controlling for age, histology, tumor grade, and stage, EC-AIA remained an independent prognostic factor associated with decreased DFS compared to EC-A (adjusted-hazard ratio 2.87, 95% confidence interval 1.44-5.70, p = 0.031). CONCLUSIONS Our study demonstrated that EC-AIA has distinct tumor characteristics and a poorer survival outcome compared to EC-A. This suggests a benefit of recognition of this unique entity as an aggressive variant of endometrial cancer.